A. Field of the Invention
The present invention relates to solid-state electrochemical gas composition control assemblies, and more particularly relates to a novel air pollution scrubber assembly for removing sulfur oxide(s) and nitrogen oxide(s) emissions from fluid streams such as from vehicular exhaust, mining, refining, industrial and manufacturing emissions, fossil fuel powered plants, flue gas, tail gas, industrial boilers, glass furnaces, natural gas driven compressors, gas turbines, catalytic cracking regenerators and the like.
Acid rain is conservatively estimated to cause more than 5 billion dollars in damages annually to crops, forests and lakes in the eastern United States alone. In 1983, the Interagency Task Force on Acid Precipitation issued its first report which clearly identified sulfur dioxide and nitrogen oxides as causal agents of acid rain.
Acid rain forms when sulfur and nitrogen oxides are converted into sulfates and nitrates in the atmosphere. Through a process known as scavenging, rainfall cleanses the atmosphere below rain clouds, removing sulfates and nitrates along with dust particles, and causing acid precipitation. In addition, when moisture in clouds coalesces to form raindrops and snowflakes, it will often do so around the nuclei of nitrate and sulfate particles. As a result of these phenomena, pollution-laden air masses capable of traveling great distances in short periods of time, can very rapidly dump massive quantities of acids in a heavy rainfall.
U.S. utilities account for more than 65% of the sulfur dioxide and 31% of the nitrogen oxide released to the atmosphere. Coal burning is the source of approximately 90% of the sulfur dioxide emissions and over 50% of the NO.sub.x (NO, NO.sub.2) emissions. Motor vehicles contribute approximately 40% of the man made NO.sub.x emissions in the United States.
The problem is not confined to the United States. Sweden imposed stringent controls after seeing 18,000 of its lakes acidified. Greece has imposed unprecedented restrictions on the use of automobiles in Athens and has forced major industrial plants to shut down during the summer months. Alarmed by government reports describing damage to 14 million acres of forests, nearly 8% of the nation's forest area, the German Interior Ministry implemented a sweeping "big furnace" ordinance, under which flue-gas desulfurization will become mandatory for nearly all furnaces with at least 100-MW thermal capacity.
Of even broader impact, the Brussels-based European Community Commission approved a directive in April of 1983 designated to control air pollution from industrial sources in Common Market countries. Under the directive, Common Market members can issue licenses to build and operate new facilities only if there would be no danger to health and no risk of major air pollution, and only if no existing Common Market or national air quality or emission standard were exceeded. The pollutants covered are SO.sub.2, NO.sub.x, heavy metals, carbon monoxide, and fluorides.
In addition to the well recognized dangers of acid rain, the principal exhaust products of vehicular exhaust, carbon monoxide, carbon dioxide, partially burned hydrocarbons, oxides of nitrogen (primarily NO), water and nitrogen, can combine in a large variety of ways in the atmosphere.
The photochemical reaction between oxides of nitrogen and hydrocarbons (HC) that caused the original interest in automobiles as a source of pollution has been extensively investigated. Ozone is a principal oxidant produced; however, comparatively low levels of some other ultimate products, such as peroxyacetyl nitrate, are apparently responsible for two unique effects of Los Angeles smog; plant damage known as silver leaf, and eye irritation.
The present invention is directed toward a gas composition control assembly which is highly effective in altering or removing volatile oxygen and/or hydrogen bearing compounds such as sulfur oxides and nitrogen oxides from fluid exhaust streams, such as from vehicular exhaust, stationary source exhaust, flue-gas and the like.
Conventional flue gas desulfurization (FGD) systems or so called "wet scrubbers" are the most expensive environmental control subsystem in a coal-fired power plant. For a new plant built today, the cost of a wet scrubber is exceeded only by the cost of the boiler itself. In a retrofit situation, the FDG cost may double. Maintenance costs are also high, ranging from 2 to 20 times higher than the rest of the plant.
These expensive systems employ an aqueous alkaline slurry, typically a lime or limestone slurry, which is sprayed on the flue gas coming from the boiler. The sulfur dioxide in the flue gas is absorbed and converted into calcium sulfite and/or calcium sulfate which is collected and disposed of.
Many FGD systems have proved to be very difficult to operate or are entirely unsatisfactory. They either fail to capture the SO.sub.2 emissions efficiently and reliably or they often become plugged and suffer severe corrosion. Nevertheless utilities will spend more than one billion dollars per year over the next decade for FGD equipment.
Because of the problems associated with the wet scrubbers, dry-scrubbing processes have been investigated. In such processes, a relatively dry alkaline powder is injected directly into the flue gas stream. The alkaline particles react with SO.sub.2 while suspended in the gas stream. Dry waste is subsequently collected in a particulate collection device (baghouse, precipitator, etc.) and the scrubbed flue gas is vented to the atmosphere. While the reactivity of dry scrubbing agents is lower than the reactivity of wet-sorbent scrubbing processes, the dry-scrubbing processes have a number of advantages. The relative absence of water minimizes cost, corrosion, erosion and freezing problems. Further, SO.sub.2 and dry particulates are controlled in a single piece of equipment. Projected capital costs are expected to be anywhere from 30 to 50% lower than wet scrubber or spray drier systems, although operating costs are expected to be equivalent. And finally, the dry-scrubbing process eliminates both the reheating requirements and high pressure drop conditions, thus resulting in a 3-5% energy savings based on plant energy.
Despite the expense and problems associated with the wet-scrubber lime and limestone systems, they continue to account for more than 90% of the utility FGD commitments, since they are usually the lowest cost flue gas desulfurization system to buy and operate, and there is a base of utility operating experience with them.
However, increasing regulatory pressures have spurred the evolution and development of several advanced flue gas desulfurization technologies which are intended to overcome one or more of the technical, economic or reliability impediments inherent in the older FGD approaches. Despite these advances, the expense and problems of reliability and efficiency place even the most advanced wet scrubber systems in the category of an interim solution.
The present invention fulfills the long-standing need for more effective technology in the field of flue gas desulfurization. The present invention is also applicable to, for example, effectively reducing the NO.sub.x emissions from stationary sources or from industrial manufacture and vehicular sources such as vehicular (automobile, truck, bus, etc.) exhaust.
A great deal of research has also been conducted on support catalysts which could effectively reduce NO.sub.x emissions from industrial, utility or vehicular sources. Out of all total NO.sub.x emissions, estimated at 20 million tons per year, combustion of fuel is by far the largest stationary and mobile source of NO.sub.x. Approximately 55% of all NO.sub.x emissions originate from stationary combustion sources, 40% from mobile sources, and the rest from chemical process industries. Combustion sources include boilers, internal and external combustion engines, gas turbines, incinerators, and the like. The use of gas turbines in electric utilities and in heavy-duty vehicular applications are expected to grow at very rapid rates and New Source Performance Standards (NSPS) for NO.sub.x are expected to become more stringent, particularly in view of the increasing concern over secondary particulate formation and acid rain.
B. Prior Art
Extensive research has been and is being carried out to cope with the problem of NO.sub.x and SO.sub.2 emission from different sources. Few, if any, satisfactory solutions have been found dealing with NO.sub.x emissions. Conventional methods and apparatus applicable to, for example, automobile exhaust emission are either inoperable, or require major power consuming modifications.
One such approach is the experimental work reported by R. Mahhaligam et al., "Catalysts Development and Evaluation in the Control of High-Temperature NO.sub.x Emissions", The American Institute of Chemical Engineers, No. 211, Vol. 77, pp 9-25 (1981). The authors describe results of passing gas mixtures containing NO.sub.x through a heated support nickel and cobalt catalyst bed 10 cm long and contained inside a 2.7 cm I.D. ceramic tube.
See also S. Pancharatram, R. A. Huggins and D. M. Mason, "Catalytic Decomposition of Nitric Oxide on Zirconia by Electrolytic Removal of Oxygen", Journal of Electrochemical Society, 122, pp 869-875 (1975); E. F. Sverdrup, C. J. Warde and R. L. Eback, "Design of High-Temperature Solid- Electrolyte Fuel-cell Batteries for Maximum Power Output per Unit Volume", Energy Conversion, Vol. 13, pp 129-141 (1973); and U.S. Pat. No. 4,253,925.
U.S. Pat. No. 4,253,925, issued Mar. 3, 1981 to David M. Mason, discloses catalytic decomposition of oxygen bearing compounds such as those contained in exhaust gases from an internal combustion engine, including NO.sub.x, CO and SO.sub.2 by the use of a solid electrolyte comprising a stabilized oxygen-ion oxide such as scandia-stabilized zirconia. The electrolyte is in the form of a solid, non-porous thin member or film. An electric field is applied across the thickness dimension by use of electrodes at opposite faces thereof. A direct current (DC) voltage source is connected to the electrodes for generation of a unidirectional electric field through the electrolyte. A very large current-limiting resistor in series with the DC voltage source is employed to limit the current drain from the source during operation.
Mason and other prior art disclosures are restricted to the use of solid non-porous stabilized oxygen-ion electrolytes. The present invention employs a highly porous, high surface area, flow-through "solid" electrolyte, preferably a stabilized oxygen-ion electrolyte. The use by the prior art of intrinsically low surface area, solid, non-porous electrolytes results in a large, heavy and ineffective device, whereas the present invention provides a viable device of compact and practical size and weight.
In addition, the Mason technology is restricted to operating temperatures of between 400.degree. to 1000.degree. C. The present invention provides a significant advance in the art as devices of the present invention are operable at temperatures of from 100.degree. C. to 2500.degree. C. This greatly expanded temperature range allows for useful functioning in environments or applications which would not be operable using the prior art systems. For example, the combustion chamber of a gas turbine operates at about 2000.degree. C., well above the upper temperature limits of Mason. Further, at idling speeds, the manifold of the gas exhaust of automobiles only reaches a temperature of about 300.degree. C., well below the lower limits of the Mason operating range.
Mason requires the anodic face of the electrolyte to be exposed to air. In the practice of the present invention, the anodic face may be exposed to air or to any environment including the exhaust stream itself. Thus, for example, the hot-probe of the present invention may be totally encased within the exhaust stream piping with no requirement for the electrolyte to be exposed to the air. This not only greatly simplifies the design of the device, but in particular, it lessens the ceramic sealing difficulties as well as the likelihood of a temperature gradient between the air and exhaust streams resulting in a cracking of the ceramic electrolyte.
Additionally, by requiring the anodic face of the electrolyte to be exposed to air, Mason precludes the utilization of the device to oxidize some components in the exhaust stream, i.e. the removal of insufficiently oxidized species such as partially burned hydrocarbons. The technology embodied in the present invention is not subject to that restriction and may be utilized to remove both reducible gases such as NO, NO.sub.2, N.sub.2 O.sub.4 and SO.sub.2 as well as oxidizable gases such as partially combusted fuels.
Another significant drawback of the Mason technology is the requirement of direct current. The present invention is not so restricted, and can employ an alternating current (AC) field as well as direct current.
Thus, the present invention may be employed to eliminate toxic effluent by the techniques of oxidation (adding oxygen atoms) and/or reducing (removing oxygen atoms), and in one preferred embodiment requires an AC field. In this novel arrangement the anode and cathode faces of the electrolyte are continuously reversing at an appropriate frequency (reversal rate) and with an optimal overall waveform. This design allows for both the oxidation (i.e. the conversion of CO, CH.sub.4 to CO.sub.2, H.sub.2 O) and reduction (i.e. the conversion of NO.sub.x, SO.sub.2 to N.sub.2, O.sub.2, S) of toxic and otherwise unwanted components in the gas stream.
Further, Mason utilizes an electric field to provide voltage-induced F-centers at the cathodic face for enhanced catalytic action. F-centers so provided for are the first stages of the electrolytic decomposition (electrolysis) of the ceramic electrolyte. This necessarily involves at least slight decomposition of the ceramic body, and a consequential strain within the body, resulting in a greatly enhanced probability of cracking the ceramic. Additionally, F-centers so provided for result in a lowering in the transference number of the electrolyte. This results in a greatly increased inefficient electronic, as opposed to useful ionic, conductivity of the electrolyte. The prior art arrangement greatly increases the input power requirements necessary to gain a useful gas removal efficiency.
Finally, the high voltages required to obtain activity (i.e. the generation of F-centers) in the prior art technology are generally above the thermodynamic reduction potentials of non-toxic exhaust gases such as water and carbon dioxide. At the voltages required to operate the Pancharatram et al. prior art device to gain NO removal, the device would require more power than the motor or turbine would actually generate, i.e. the parasitic power consumption of the prior art will tend to exceed 100% of the power produced by the engine, motor or turbine.
The two above-cited Pancharatram et al. and Sverdrup et al. references essentially disclose the device of Mason without the megaohm sized current limiting resistor employed by Mason in the external electrical circuit as an improvement over the prior art.
Thus, Mason does not provide an electrochemical process, but rather discloses a catalytic process wherein the intrinsic catalytic properties of the materials are increased by the application of an electric field. This alteration from an electrochemical process to a catalytic process limits the device to the removal of gases which would otherwise be spontaneously removed from the gas stream, i.e. that have positive Gibb's energies. On a practical basis, the Mason technology would be restricted to the elimination of compounds which have positive, intrinsically unstable Gibb's energies and would preclude utility against such materials as SO.sub.2 which have a negative, intrinsically stable Gibb energy.
All-in-all, a Mason type non-porous electrolyte based device is impractical and inferior to the advance in the art provided by the present invention.
See also U.S. Pat. Nos. 3,755,120; 3,180,083; 2,998,308; 2,938,593; and 2,928,593.
In all, the literature reports that more than 600 NO.sub.x catalysts on various supports have been developed and evaluated on automotive exhaust systems. Compounds of 36 individual metals or combinations have been used to make catalysts on about 20 different supports. Nevertheless there continues to be a substantial need for improved, reliable, efficient, more effective, economical devices which can operate over a wide range of temperatures and reliably remove SO.sub.2, NO.sub.x, and the like from exhaust, regardless of the source thereof.
While advances in the field have been achieved, there exists a need for the efficient treatment of exhaust gases, in particular to control both SO.sub.2 and NO.sub.x emissions reliably, efficiently and in a cost effective manner over a wide variety of operating systems. The present invention meets that need.
For example, the set up costs for a 500 megawatt boiler using the technology of the present invention is approximately 90% less than that of the leading competitive technology for flue gas treatment, the Gleason et al electron beam technology. Over a twenty five year period, there is an estimated savings of approximately one billion dollars compared to electron flue gas treatment technology for a single power plant.